Recently published by the American Association for Laboratory Accreditation (A2LA) are Guidelines for Estimating Uncertainty for Microbiological Counting Methods.

Showing three worked examples the guidelines are intended to demonstrate ways to implement the A2LA policies for the estimation of measurement uncertainty for methods that use counting for determining the number of colonies in a test sample.

These examples apply for all quantitative microbiological methods, including plate count and MPN. To the extent possible they use conventional concepts for estimating uncertainty, but they are not the only possible methods for determining uncertainty of counting methods.

These examples are based on 20 data points each, but larger or smaller data sets may be used. The coverage factor obtained from the Student-t tables should be adjusted to reflect the data set used.